make options for credo configurable (#2337)

* Add credo --strict option

If a user sets 'let g:ale_elixir_credo_strict=1' it will run credo with
--strict instead of suggest.  The default (0) is to run as suggest.

* Added credo docs

doc/ale-elixir.txt

@@ -72,6 +72,18 @@ g:ale_elixir_elixir_ls_config                   *g:ale_elixir_elixir_ls_config*
     \ }
 <
   Consult the ElixirLS documentation for more information about settings.
+===============================================================================
+credo                                                        *ale-elixir-credo*
+
+Credo (https://github.com/rrrene/credo)
+
+g:ale_elixir_credo_strict                           *g:ale_elixir_credo_strict*
+
+  Type: Integer
+  Default: 0
+
+  Tells credo to run in strict mode or suggest mode.  Set variable to 1 to
+  enable --strict mode.
 
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale.txt

@@ -119,6 +119,7 @@ ALE supports the following key features for linting:
       mix_format..........................|ale-elixir-mix-format|
       dialyxir............................|ale-elixir-dialyxir|
       elixir-ls...........................|ale-elixir-elixir-ls|
+      credo...............................|ale-elixir-credo|
     elm...................................|ale-elm-options|
       elm-format..........................|ale-elm-elm-format|
       elm-make............................|ale-elm-elm-make|
@@ -2256,6 +2257,7 @@ documented in additional help files.
     mix_format............................|ale-elixir-mix-format|
     dialyxir..............................|ale-elixir-dialyxir|
     elixir-ls.............................|ale-elixir-elixir-ls|
+    credo.................................|ale-elixir-credo|
   elm.....................................|ale-elm-options|
     elm-format............................|ale-elm-elm-format|
     elm-make..............................|ale-elm-elm-make|